Gene Ecology of the Climbing Common Bean (Phaseolus Vulgaris) - Bean Common Mosaic Virus/ Bean Common Mosaic Necrosis Virus (BCMV/BCMVN) Relationship in Rwanda

Download or read book Gene Ecology of the Climbing Common Bean (Phaseolus Vulgaris) - Bean Common Mosaic Virus/ Bean Common Mosaic Necrosis Virus (BCMV/BCMVN) Relationship in Rwanda written by Alice Kabeja and published by . This book was released on 2020 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Common beans (Phaseolus vulgaris) are nutritionally, culturally, and economically an important legume crop in Rwanda. Beans provide 32 and 65 percent of calories and protein intake in the Rwandan diet, whereas protein sourced from animal provides only 4 percent of the protein intake. Unfortunately, common bean production is constrained by multiple biotic and abiotic factors in this country. Among others, Bean common mosaic virus and Bean common mosaic necrosis virus (BCMV/BCMNV) are seedborne viral pathogens that are the main constraint in common bean production. The only effective control of these pathogens is through the use of genetic resistance. Currently, the status of BCMV/BCMNV in Rwanda is not known. To contribute to the research in breeding common bean against BCMV/BCMNV in Rwanda, I sought to better understand the variability and biology of the BCMV/BCMNV pathogen to assist in developing a resistance strategy in common bean that is broader and stronger. In the first chapter, I was interested in determining the types of BCMV/BCMNV strains that are present in Rwanda. I was also interested in identifying the selective pressures these viruses are subjected to and how they govern the evolution of these viruses in Rwanda. Furthermore, I wanted to find out if there are alternate hosts other than common bean for BCMV/BCMNV in Rwanda. To gain this information, I surveyed common bean fields extensively in high-altitude areas of Rwanda. The biological and molecular pathogenic characterization studies of isolates of BCMV and BCMNV resulting from these surveys revealed the exclusive presence of BCMNV in Rwanda. No BCMV strain were found. We observed the predominance of the NL-3 strain of BCMNV. The study of genetic diversity for coat protein (CP) fragment of the BCMNV isolates showed a low genetic diversity ([pi] = 0.015623). The mean [omega] (dN/dS) value for CP of 27 isolates analyzed here was less than 1.0, indicating that the CP segment of BCMNV in Rwanda is subject to negative or purifying selection. No alternative hosts to Phaseolus vulgaris for either BCMV or BCMNV were identified in this study. For now, the resistance of BCMV/BCMNV in host plant is controlled by seven resistance genes: one dominant gene (I gene), five recessive strain-specific genes (bc-1, bc-12, bc-2, bc-22, and bc-3), and one strain-nonspecific epistatic gene (bc-u). However, few studies have been carried out on bc-12 (Miklas et al.; 2000). In the second chapter, through the development and evaluation of Recombinant Inbreed Lines (RIL) under greenhouse conditions, we investigated if any minor, quantitative loci could be identified for resistance to BCMNV, which would have originated in either of the parents, RWV3006 (bc-12) or Mshindi (bc-12), and would complement the major resistance gene bc-12. A quantitative trait loci (QTL) study done on these RILs confirmed the presence of bc-12 gene (SS715647636 QTL) on chromosome PV03 as expected. However, another QTL (SS715645701) with a smaller effect was found on Pv09. Interestingly, the positive (resistant) allele at this QTL came from RWV 3006, the susceptible parent. Results from this study showed that the combination of both QTLs increases resistance, either at the heterozygous or homozygous state. It sets the stage for further surveys of minor viral resistance genes in common-bean germplasm. To facilitate future breeding efforts, in the third chapter I attempted to obtain tightly-linked markers useful for marker-assisted selection (MAS) of the bc-12 gene in the presence of other epistatic virus resistance genes. These markers should be more broadly applicable to different genetic backgrounds, reproducible across laboratories, and able to distinguish homozygotes from heterozygotes (co-dominant). Based on a known SNP marker linked to bc-12 resistance gene, we developed a dCAPS (derived Cleaved Amplified Polymorphic Sequence) marker. However, it was not possible to clearly visualize the polymorphism generated by the designed dCAPSHindIII marker on agarose gels in all tested genotypes. This may be explained by the small size of the marker. The low resolution of the agarose gel could not detect small size differences as effectively as polyacrylamide gel electrophoresis, which can distinguish fragments with few (one to five) base pairs difference. We suggest that the marker should be tested in a larger number of different bean seed types to ascertain its usefulness in different bean classes. Furthermore, in future work with this dCAPSHindIII marker, the use of polyacrylamide gel could facilitate effective discrimination between DNA fragments from the resistant and susceptible parents.